This invention relates generally to electric motors and, more particularly, to a line start magnetically salient rotor AC electric motor.
Line start permanent magnet motors include rotors having permanent magnets and induction squirrel-ages. The induction squirrel cages enable starting on a conventional AC power source, and the permanent magnets improve motor efficiency. Such rotors sometimes are referred to herein as divided magnet rotors.
In an exemplary form, a divided magnet rotor includes a rotor core, a rotor shaft, permanently magnetized locations, and secondary conductors. The rotor shaft extends through the rotor core and is coaxial with the rotor core axis of rotation. The secondary conductors also extend through the rotor core and are arranged axially with respect to the rotor shaft. Such secondary conductors are offset from the outer circumference or periphery of the rotor core and are connected at opposite ends of the core by end rings. Notches at the outer periphery of the rotor core typically are radially aligned with at least one secondary conductor. Permanent magnets are located in the notches and the permanent magnets are magnetized to form a selected number of poles.
To decouple stator slot order harmonics, the rotor bars in the squirrel cage typically are skewed. Skewing is accomplished by slightly turning the rotor laminations with respect to each other so that the passages formed by overlapping slots of the rotor laminations are generally helical in shape. In a divided magnet rotor, skewing the laminations is difficult. Particularly, some magnetic materials that may be used for the permanent magnets are brittle and prevent such skewing.
In addition, open slot rotors generally provide advantages over closed slot rotors. In a closed slot rotor, the flux flows through the bridge (i.e., the region of iron immediately towards the rotor outer diameter from the rotor bar) and saturates the bridge depending on the rotor current. The level of current at which the bridge saturates will be passed through four times per cycle, causing time harmonics in the stator current. These time harmonics create the basic forcing function for a class of noise. The leakage flux which causes the bridge to saturate reduces the torque produced by the machine at that current level and in turn raises the losses related to current flow at a give torque. An open slot rotor does not provide a high permeability path for this component of the leakage flux. Open slot rotors, however, typically are more difficult to fabricate than closed slot rotors.
It would be desirable to provide a divided magnet rotor that includes permanent magnets yet also decouples stator slot order harmonics. It also would be desirable to provide such a rotor which is not subject to rotor bridge saturation.
In an exemplary embodiment of the invention, a divided magnet rotor includes a stepped skew rather than a helical skew. The stepped skew enables the use of straight magnet sections that can be inserted into the rotor core notches thereby eliminating the need to produce a helix from the rotor cage. The stepped skew is effective in decoupling stator slot order harmonics. In addition, the stepped skew rotor includes, in some embodiments, open slots so that the rotor is not subject to rotor bridge saturation.
The divided magnet rotor includes a rotor core, a rotor shaft, permanently magnetized locations, and secondary conductors. The rotor shaft extends through the rotor core and is coaxial with the rotor core axis of rotation.
The rotor core includes rotor laminations in a stack arranged in at least two sets. The slots in the first set of laminations have skew portions extending laterally in a first direction, and the slots in the second set of laminations have skew portions extending laterally in a second direction opposite the first direction. The radially inner portions of corresponding slots in the first and second sets of rotor laminations overlap each other. Such a configuration forms a stepped skew.
Notches, or channels, extend from an outer diameter (OD) of the rotor laminations to the skew portion of each respective slot. The notches extend axinay, and permanent magnets are located in the notches. Specifically, straight magnet sections of permanently magnetized material are inserted into the notches. The straight magnet sections are magnetized to form a selected number of poles. The secondary conductors extend through the rotor core slots and are arranged axially with respect to the rotor shaft. The secondary conductors are offset from the outer circumference or periphery of the rotor core and are connected at opposite ends of the core by end rings.
The above described divided magnet rotor has a stepped skew rather than a helical skew. The stepped skew enables the use of straight magnet sections that can be inserted into the rotor thereby eliminating the need to produce a helix from the rotor cage. The stepped skew is effective in decoupling stator slot order harmonics. In addition, the rotor has open slots so that the rotor is not subject to rotor bridge saturation.